1. Field of the Invention
The present invention relates to ultrasound imaging, and more particularly, to a method and apparatus for improving and enhancing ultrasound images.
2. Description of the Background Art
Ultrasonic imaging is frequently used for a variety of diagnostic procedures due to its non-invasive nature, low cost, and fast response time. These qualities are especially true in medical fields where the added benefit is reducing or eliminating a patient's exposure to radiation. Typically, ultrasound imaging is accomplished by 1) generating and directing an ultrasonic beam into media under investigation; and 2) observing any resulting waves that are reflected back from dissimilar tissues and tissue boundaries within that area. The resulting waves are received as signals. These received signals are then post-processed and imaged on a screen by plotting a spot whose intensity is proportional to the amplitude of a reflected beam from a given location. Determination of location is based upon a known transmission and re-radiation rate after the ultrasonic wave is pulsed into the media under investigation.
Typically, an ultrasonic signal transmitted into the media under investigation includes a burst of sinusoidal waves of a given waveform. These sinusoidal waves are applied to a transducer and form a transmitted signal. The transmitted signal is typically in the range of 40 kHz to 50 MHz, but more commonly, in the range of 40 kHz to 1 MHz. As the transmitted signal interacts with tissue layers and boundaries between layers, the ultrasonic signal is modified by being scattered, resonated, attenuated, reflected, or transmitted.
Media under investigation are often a non-linear media such as those commonly found in the human body. Non-linear media produce harmonic frequencies in the echoed signals. These additional frequency components continue to re-radiate through and, in turn, reflect off or interact with other structures. A portion of the reflected (or echoed) signals propagates back to a receiving transducer.
Fundamental and harmonic frequencies that are impinged upon a receiving transducer includes the full signal, which must then be further processed to eliminate noise and extraneous components. The receiving transducer may be the same as a transmitting transducer, or can be completely independent. When the same transducer is used, a transmit/receive (T/R) switch connects the transducer to either the transmitter electronics or the receiver post-processing electronics. The receiving transducer accepts the echo signal plus any generated noise and furnishes these to a portion of the post-processing electronics known as a beam former. Beam formers reject noise and have either an adaptive or fixed configuration. Adaptive beam formers are designed to reject variable directional noise sources by monitoring the noise field and adjusting internal parameters to minimize the background noise. Fixed beam formers are designed to reject isotropic noise and take advantage of the directional property of the reflected signal.
Ultimately, ultrasonic images of the human body are a product of harmonic imaging. Harmonic imaging is generally associated with either visualization of tissue boundaries and densities of different media, or imaging contrast agents at harmonic frequencies. Contrast agents are typically fluid filled micro-spheres that resonate at ultrasonic frequencies. Such agents are injected into the blood stream and are carried to various parts of the body. Once these agents are pulsed at ultrasonic frequencies, harmonic echo-locator signals are generated due to the resonance produced within the micro-spheres.
While ultrasonic procedures have a distinct number of advantages over other types of diagnostic techniques, prior art methods and systems have noise problems that make it difficult to determine the exact location and proper interpretation of the received signal. Various forms of averaging techniques have been employed to reduce the noise, but averaging alone is ineffective in locating images of interest between tissues with similar densities. (Echoed signals from tissues with similar densities will indicate a uniform mass with indistinct boundaries. Averaging won't help in this situation.) These interpretation difficulties are exacerbated by the fact that many tissues in the human body have similar densities. Therefore, a method and system are needed that can effectively overcome the stated difficulties while not negating the positive benefits of ultrasound imaging systems in general.